1. Field of the Invention
The present invention relates to corrosion inhibition and more particularly to inhibition of corrosion in environmentally sensitive aqueous media.
2. Description of Prior Art
Corrosion of metal surfaces in aqueous media has long been an intractable problem. This problem is especially troublesome in deep sea operations such as off-shore oil production, where corrosion inhibitors must satisfy several criteria in order to be effective in the demanding conditions encountered in such operations. A number of corrosion inhibitors have been developed in attempts to satisfy the demands imposed by such activities. However, because it is difficult to meet each of several independent corrosion inhibition criteria, these efforts have met with varying, but not totally satisfactory, success.
Nevertheless, increasing environmental concerns have introduced even further criteria for corrosion inhibitors to satisfy. In particular, the corrosion inhibitor should be compatible with the sensitive life forms indigenous to the medium into which the inhibitor is incorporated.
For example, in North Sea operations, survival not only of fish, but also, for instance, of algae and the microorganism Skeletonema costatum is of concern. Thus, environmental constraints have been imposed on the types of compositions used in the North Sea, thereby to protect such organisms. However, commonly available commercial inhibitors have been found to be too toxic to the organism. More specifically, even a concentration of less than one part per million by weight (ppm) of conventional inhibitors has been found to retard growth of Skeletonema costatum test populations by 50% in 96 hours. This may be written as EC.sub.50 &lt;1 ppm. Thus, a corrosion inhibitor having an EC.sub.50 greater than 1 ppm is desired.
In addition, it is desired that the inhibitor meet several other environmental criteria as well. For example, the inhibitor should be sufficiently biodegradable so that within 28 days after treatment, the inhibitor has degraded at least 70% in terms of the theoretical oxygen consumption required for complete degradation (i.e.; the biochemical oxygen demand BOD-28.gtoreq.70%). Of course, it is more desirable that complete degradation occur within 28 days. That is, the goal is BOD-28=100%.
Further, the water-solubility of the inhibitor should be sufficient to avoid or to minimize bio-accumulation that otherwise can result with fat soluble inhibitors in lower life forms. The fat soluble inhibitors may become more concentrated as they move up the food chain. This may be quantified by measuring the resulting concentration of inhibitor in the octanol phase and in the water phase of an n-octanol/water medium into which the inhibitor has been injected, and dividing the former by the latter. It is desired that the logarithm (base 10) of that quotient be less than three. Stated another way, log K or "partitioning" should be less than three in terms of a log value.
Moreover, because evaporation of a toxic solvent (if any) would be undesirable, the Solvent evaporation factor (YL) should not be greater than three. And, because of the dangers of flammability, the flash point should be greater than 56.degree. C.
Another problem that has been encountered is the tendency of some inhibitors to form a floc with calcium ions that may be present in the medium to be treated, clouding the medium and short-circuiting the effectiveness of the inhibitor. Accordingly, the inhibitor should not tend to form a floc with calcium ions as well as meet the significant efficacy and environmental criteria.
Commercial inhibitors generally have not been found to meet such demanding criteria, especially while still affording highly effective corrosion inhibition. Thus, inhibitors that not only provide satisfactory corrosion inhibition, but satisfy such environmental concerns as well, are still being sought. In fact, because of increasing environmental emphasis, the search is intensifying.